1. Field of Invention
This invention relates to a method of identifying a particular optical cable out of a number of similar optical cables on an installation site.
2. Prior Art
In modern telecommunication systems, optical cables are housed in ducts and conduits and laid along the route of installation.
With the recent development of telecommunication networks, a large number of different optical cables are often laid along a single route of installation to produce a congested condition of cables. Works for branching and/or replacing the installed cables are frequently required.
FIG. 4 of the accompanying drawings schematically illustrates a typical telecommunication network comprising central stations O1, relay stations O2 and terminal stations O3 interconnected by means of a large number of optical cables C, which are often ramified, looped and star-connected.
Since many of these optical cables resemble one another, skilled workers feel it difficult to single out a particular optical cable that needs to be branched or replaced out of a number of cables that have been laid in a same route of installation.
In an attempt to remedy this problem, Laid-Open Japanese Patent Nos. 2-230105 and 1-230106 propose methods of identifying a particular optical cable by utilizing fluctuation of polarized light in a single mode optical fiber.
Referring to FIG. 5, with these known methods, polarized light is transmitted through a single mode optical fiber of an optical cable C from a light source 1 of a light source station while an external signal (such as mechanical vibration) is applied to the optical cable C at a work site by means of a signal application device 2 so that any fluctuation in the level of polarized light caused by the external signal may be detected to identify the optical fiber by a photodetector 3 as the light is received by a light receiving device 4 at a detecting station.
More specifically, while the operator of the light source station applies a given signal to each optical cable and the operator of the detecting station monitors the reception of the signal, the operator on the work site sequentially applies an external signal to the optical cables laid there on a one by one basis and, each time an external signal is applied to an optical cable, the operator at the work site and his colleague at the detecting station talk over through a radio communication channel, using, for instance, a pair of transceiver sets, to make sure if the optical cable in question is detected or not.
As the operation of sequentially applying an external signal to the optical cables laid in the work site proceeds on a one by one basis, the operator there will eventually come across the optical cable in question to apply a signal to it and the operator monitoring the signals at the detecting station will detect the cable carrying a fluctuated signal.
Then, the operator at the detecting station notifies his colleague at the work site that the cable to which an external signal is applied last time is the optical cable in question so that the latter can identify the optical cable.
Now, the optical cable is identified and the operator on the work site can proceed to a predetermined work to be conducted on the cable in question.
Since the above described method of utilizing fluctuation of polarized light involves mechanical vibration which is applied to optical cables as an external signal, the applied external vibration can affect the optical cables laid in the vicinity of the source of vibration to give rise to a phenomenon of cross talk.
In other words, the external signal applied to a particular optical cable can also be given to any of the optical cables laid in the vicinity and produce a condition where identification of cables is impossible or misidentification of cables occurs.
Since the phenomenon of cross talk appears when cables are brought to contact with each other, it may seem that such a phenomenon can be avoided when cables are laid in a manner that eliminates any possibility of mutual contact.
However, laying optical cables under a condition where the possibility of mutual contact is completely eliminated is illogical and irrational from the view point of boosting high density cable installation that has encouraged the development of optical cables.
Besides, in view of the optical cables of existing cable networks that stretch over hundreds of thousands kilometers, it is highly unrealistic to rearrange them under a non-contact condition.
Consequently, it is impossible under the current condition of optical cable installation to totally eliminate the possibility of misidentification of cables when the above described technique of utilizing fluctuation of polarized light is used for identification of particular cables.